The relative thermal stability of jet fuel is an important factor in practice. The standard test for this property, the JFTOT method (ASTM D3241), is used as a qualitative test to determine whether the fuel can be used. This test method covers the procedure for rating the tendencies of gas turbine fuels to deposit decomposition products within the fuel system. The fuel is tested at a required minimum temperature to determine if it meets minimum acceptance criteria. The JFTOT test basically determines whether the fuel passes or fails the test based on the breakpoint of the fuel. The breakpoint method requires running multiple JFTOT tests with an equivalent multiplication of time. The test usually takes about three hours. The breakpoint of the fuel is defined as the highest temperature in degrees Centigrade at which the fuel receives a passing rating. For example, the minimum JFTOT breakpoint for salable jet fuel is 260 degrees Centigrade.
While the JFTOT has proven successful in ensuring the delivery of quality jet fuel, there has always been a desire for a quicker screening test especially for storage and transportation monitoring. If there is a suspicion of a problem, the JFTOT is employed but it takes too much time for routine analysis. A quick, routine screening tool could eliminate needless testing on otherwise good fuel and ensure testing of potentially bad fuels that would have been overlooked.
U.S. Pat. No. 5,198,871 discloses the use of laser induced fluorescence (LIF) to measure the presence of heavy molecules (deposition precursors) and evaluate the quality of fuels. In U.S. Pat. No. 5,198,871, a sample is illuminated to cause it to produce fluorescent radiation. The spectral representation of the fluorescence produced by the sample is compared to a reference spectrum to obtain an indication of the physical characteristics of the sample based on the supposition that the fluorescence is proportional to the amount of the precursor present in the sample for a given fuel.
However, U.S. Pat. No. 5,198,871 provided no correlation between the fluorescence and the thermal stability. Furthermore, fuels may naturally fluoresce, some more than others. Moreover, past researchers have attempted to describe the chemistry of deposition with limited success. Specifically, the past researchers developed models that were not universally applicable.